Plywood = warp due to unequal stresses in the wood layer sandwich. OSB = less warp due to random orientation of stresses, but it falls apart after too much fun in the sun.Polystyrene = works great as long as the ants don’t find it.

Most of my hive tops are polystyrene. However you still need something under those tops or the bees will try to glue the poly tops down to the hive body. I use a sheet of plastic (like 3mil stuff) for my inner covers. Bees propolis doesn’t stick to polyethylene. Polystyrene also lacks protection from UV. It needs to be painted/coated with something or else the UV will turn about 4mm of it to dust every year until it is all gone.

I think one of the joys of beekeeping is the woodworking that is required. I'm learning that bee's really LIKE wood and all the componets that go along with that. If plywood is used with the pine framing, with the correct joints and glue, painted and protected from the elements (mainly moisture) then even 1/4 will lay straight and last for years. yes...I even like telescoping covers for a nuc......protects it and you can use it over and over for many years. While I dont have many beehives, I've built many other things over the years thats sitting outside and if done right.....they last. Of course I'm not building hundreds of these so I dont have that worry.